1. Field of the Invention
The present invention relates to a stainless steel flux-cored wire for arc welding filled up with flux in an outer sheath made of stainless steel, and relates specifically to a stainless steel flux-cored wire suppressing the amount of hexavalent chromium included in fume generated in welding stainless steel.
2. Description of the Related Art
In general, in slag and fume generated in welding stainless steel, Cr is included by 10 mass % or more, and when the slag and fume are dumped to soil and the like as they are and left for a long period of time, there is a problem that Cr may possibly be eluted into the soil and the like as hexavalent chromium (Cr6+).
Recently, the concern over the industrial waste is increasing year by year as one of the environmental problems, and the permissible value of the hexavalent chromium amount in the ordinance issued by the Prime Minister's Office of Japan stipulating the criteria in relation with the industrial waste including metal and the like (Ordinance No. 5 issued on Feb. 17, 1973; as amended thereafter) is stipulated to be 1.5 ppm or below in land-fill disposal, and 0.5 ppm or below in a waste water treatment. Also, hexavalent chromium is reported to be a substance causing the damage of respiratory organs due to inhalation exposure and having carcinogenicity, and is extremely harmful to a human body. Therefore, in order to prevent elution of hexavalent chromium, prior arts described below have been proposed as the flux-cored wires for welding stainless steel.
In Japanese Published Unexamined Patent Application No. 2003-320480, a flux-cored wire is disclosed which contains Si: 1.0-4.0 mass % and Cr: 16-30 mass %, and Si/(Ti+Zr) is 0.8 or above and (Na+K)×Cr2 is 50 or below.
In Japanese Published Unexamined Patent Application No. 2007-50452, it is disclosed that elution of Cr can be suppressed when the Mn content in an elution test liquid produced by mixing fume generated when stainless steel is welded using a flux-cored wire for welding stainless steel with distilled water of the mass of 100 times of the mass of the fume is 70-220 mass ppm, and pH of the elution test liquid is 5.8-7.8.
In Japanese Published Unexamined Patent Application No. 2009-154183, a wire for welding stainless steel is disclosed which, in order to suppress elution of hexavalent chromium from slag, contains Cr: 12-32 mass %, N: 0.005-0.06 mass %, Ca: 0.01 mass % or below, Na: 0.01-0.5 mass %, K: 0.01-0.5 mass %, Na+K: 0.01-0.5 mass %. Also, in this document, it is disclosed that the wire is annealed by hydrogen gas in the manufacturing process of the wire.
However, in the fume generated in welding stainless steel, Cr is included by a great amount, and a part of Cr in the fume is present as hexavalent chromium (Cr6+). In recent years, the harmful effect of hexavalent chromium has been re-evaluated and its restriction value has been made stricter. For example, the Occupational Safety & Health Administration (OSHA), United States Department of Labor made the permissible exposure limit (PEL) of hexavalent chromium stricter in 2006 from 52 μg/m3 to 5 μg/m3, or about 1/10 of the conventional value, which came into effect in June 2010. In response to it, as a method that can analyze hexavalent chromium more sensitively, a method combined with the hexavalent chromium extracting ion chromatograph and the post column color developing method was developed, and became an international standard as ISO 16740:2005. According to this analytical method for hexavalent chromium, an alkaline solution is adopted for hexavalent chromium extraction of the pre-treatment, and hexavalent chromium can be extracted more stably. In the flux-cored wires for welding stainless steel described in Japanese Published Unexamined Patent Application No. 2003-320480 and Japanese Published Unexamined Patent Application No. 2007-50452, hexavalent chromium in fume is analyzed by the diphenyl carbazide absorption method after extraction treatment by distilled water, however when the analytical method of ISO 16740:2005 is employed, even when the composition is in the range described above, there is a case that sufficient hexavalent chromium reduction effect cannot be secured. Further, because the weldability also has not reached a practical level, these flux-cored wires have not been practically used.
Also, the flux-cored wire for welding stainless steel described in Japanese Published Unexamined Patent Application No. 2009-154183 aims to reduce hexavalent chromium in slag, and has a problem that it is not effective in reducing hexavalent chromium in fume.